Carlson in U.S. Pat. No. 4,064,521, incorporated herein by reference, has disclosed amorphous silicon devices wherein the semiconductor materials are deposited from a glow discharge in a monosilane gas. These devices are useful in high-efficiency photodetectors, thin film transistors and electrophotography.
Experimental studies have shown that a high-quality amorphous silicon semiconductor layer can be deposited at a high deposition rate from a glow discharge in disilane. However, commercially available disilane is more expensive than monosilane and may be contaminated with impurities such as chlorosilanes, siloxanes and alkyl hydrocarbons from the manufacturing process. Disilane has been produced in low yields by the chemical reaction of metal silicides with inorganic acids, the reaction of hexachlorodisilane with lithium aluminum hydrides and by silent discharge methods from monosilane. The silent-discharge method, as exemplified by Spanier et al. in the Journal of Inorganic Chemistry 1, 432 (1962) and Gorkale et al. in the Journal of Inorganic Nuclear Chemistry 27, 1911 (1965), comprises passing monosilane gas through an electrical discharge in an ozonizer. The ozonizer comprises a chamber having a pair of closely spaced electrodes with a dielectric therebetween. A silent discharge between the electrodes induces a diffuse high pressure glow on the surface of the dielectric which is often described as a brush discharge and which stimulates the conversion reaction of a simple hydride to a complex hydride. The silent discharge is distinguished from the low pressure glow discharge used by Carlson to deposit an amorphous semiconductor layer in that the gas pressure in the ozonizer is much greater than about 0.1 atmosphere and the inter-electrode spacing is much smaller. Correspondingly, a low pressure glow discharge cannot be sustained in an ozonizer because of the small inter-electrode spacing. The ozonizer produces disilane from monosilane with a low efficiency, typically less than several percent per passage through the ozonizer, thus requiring the use of low-temperature traps to separate the higher silanes and a multi-pass system for recirculating the remainder of the monosilane through the ozonizer until a useful concentration of the disilane is obtained.
Disilane has also been observed in diagnostic studies of the glow discharge in monosilane used for the deposition of amorphous silicon according to the process disclosed by Carlson. In these studies the concentration of disilane in the discharge was found to be less than about five percent.
It would thus be clearly desirable to have a high-efficiency means for producing comparatively pure disilane in order to improve the process for the deposition of an amorphous silicon layer therefrom.